Elevated Floating Bridge on Washington State Route 520

The Washington State Department of Transportation (WSDOT) is reconstructing the Evergreen Point floating bridge, which runs over Lake Washington along State Route 520 (SR 520) near Seattle, to better accommodate varying winds and storms. The new design will be elevated, which will better protect drivers and the road from waves and storms, and the pontoons that allow the bridge to float will be designed to withstand higher winds. In its climate impacts vulnerability assessment, WSDOT has recognized that high winds could increase as a result of climate change and more extreme weather events.

Floating bridges are held up by large water-tight concrete pontoons, upon which the roadway is built. The pontoons are connected to each other and held in place by steel cables that are anchored deep in the lakebed. The Evergreen Point Bridge contains a 1.44 mile-long floating section, making it the longest floating bridge in the world. However, the existing bridge is aging and vulnerable to high winds and waves during storms. The existing bridge was designed to withstand 50-70 mph winds; currently, WSDOT closes the bridge whenever gusts of wind reach 50 mph and are sustained for 15 minutes. For example, in 2006 the bridge was closed during peak afternoon traffic due to a severe windstorm. Strong winds could cause the drawspan, anchor cables, or pontoons to break or crack, causing the bridge to sink. Crews from WSDOT have already repaired over 30,000 linear feet of cracks in the existing bridge pontoons since a severe storm occurred in 1993. Storms often also cause large waves on Lake Washington to beat against the southern side of the bridge and onto the bridge deck, creating additional traffic safety concerns.

To better withstand variable and possibly increasing winds, the new bridge design will be built to resist windstorms up to 89 mph. The bridge deck will also be elevated approximately 7 feet higher than the existing design – a total of about 20 feet from the water surface. The elevated design will allow drivers to travel safely across the bridge even during wind conditions up to 70 mph and avoid interference from waves on Lake Washington that could otherwise hit against and wash up onto the bridge deck. Elevating the bridge deck will also minimize bridge closures during maintenance work on pontoons, and will better accommodate future light rail should it be constructed along SR 520. The floating bridge is also being constructed according to current earthquake design standards; the west approach of the bridge will be able to withstand a 1,000-year earthquake event.

The new floating bridge will open to drivers in spring 2016, and the old floating bridge is expected to be removed later in 2016. Replacement of the floating bridge is one of several projects within the SR 520 Bridge Replacement and HOV Program. WSDOT has prioritized the projects in order of the most vulnerable components of the SR 520 corridor, with the floating bridge of highest priority due to the frequency of severe storms and risk of catastrophic failure. The SR 520 Program is designed to enhance safety and mobility along the entire 12.8-mile SR 520 highway, which extends from Seattle to Redmond, and is anticipated to cost $4.3 billion. Funding for the SR 520 program is provided by a variety of state and federal sources, including the addition of tolls on the existing SR 520 floating bridge starting in December 2011.

 

This Adaptation Clearinghouse entry was prepared with support from the Federal Highway Administration. This entry was last updated on March 22, 2016.

 

Publication Date: Spring 2016

Related Organizations:

  • Washington State Department of Transportation

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  • Case study

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